Upgrade PayPal Integration to V6

I’m running an e-commerce website that processes payments through PayPal, but the current integration is on an older version of the API. The site is a custom build that sits on an Apache stack, so nothing is tied to a boxed-in CMS like Shopify or Magento... (Budget: €30 - €250 EUR, Jobs: Apache, API Development, eCommerce, JavaScript, Magento, Payment Processing, PayPal, PHP)